This invention relates to drill pipes for rotary-vibratory drills and, in particular, to joints for the drill pipes.
Rotary-vibratory drills employ a vibratory force superimposed upon a rotary action to accomplish the drilling operation. Sonic drills are rotary-vibratory drills where the vibration is in the sonic range. Sonic drills are used for such applications as drilling through overburden in placer exploration, installing concrete piles, water well drilling, rock drilling for blast holes, for rock coring and for testing soil for levels of contamination.
One of the major problems associated with sonic drilling machines in the past has been the failure to develop suitable drill tooling. Extremely high alternating forces are generated within the drill pipe. Standard drill pipe is designed to withstand the torque developed during rotary drilling, but not the high alternating tensile and compressive loads encountered in sonic drilling. These reversing loads are especially critical at the threaded ends of tool joints because of the stress concentration created by the presence of the threads.
Various approaches to solving the problem of joint failure have been attempted. One was to machine threads directly onto the drill pipe. This was not completely successful because the threads reduced the cross-sectional area of the pipe and simultaneously acted as a point of stress concentration as suggested above.
Another attempt was to use heavy wall pipe. However, it was then realized that the drill pipe stains or elongates and contracts the same amount in resonance regardless of its thickness. The greater cross section simply increases the force which must be transmitted by the joint, leading to failure at the threads.
Next, drill pipe was tested with a relatively thick tool joint in relation to the thickness of the drill pipe. This configuration is routinely used for rotary drills used in drilling oil and water wells. These are generally assembled by circumferential welds between the pipe and threaded connection members used at the joint or by friction welding the members to the pipe. These welds however do not long withstand the fatigue loading conditions and high stress concentrations encountered in sonic drilling.
Pipe joints employing alternating fingers and slots extending circumferentially about the pipe have been used for other purposes such as the oil drilling platform disclosed in U.S. Pat. No. 3,521,811 to Bardgette. However, the problem encountered in that instance is not analogous to the difficulties encountered in sonic drilling and therefore does not suggest a solution to the problem.
The problems were largely overcome by my earlier U.S. Pat. No. 5,086,854 which disclosed a drill string where the drill pipes are welded to threaded connectors along V-shaped slots in the drill pipe. However, although a vast improvement over the prior art, some problems remained. In particular, the life of the drills has been shorter than considered desirable.
Analysis has shown that the problem is caused by the weld puddle at the bottom of the V-shaped slot in the drill pipe. As this weld puddle cools, it shrinks and causes high tensile stresses that this location. When the pipe is then vibrated, cracks form around these weld puddles. Eventually the cracks start to propagate until they meet each other and the pipe fails completely. If this happens in a deep hole, then the length of pipe below the fracture point is lost at considerable expense.
The stress concentration caused by the shrinking weld puddle theoretically could be reduced or eliminated by subjecting the entire length of the pipe to a stress relief procedure. However this is not very practical since stress relief furnaces are usually not long enough. Furthermore, only a few pieces could be done at one time since the rods and casing would go too far out of round unless they are properly supported. Consequently such stress relieving would make the parts too expensive.
Accordingly it is an object of the invention to provide an improved rotary-vibratory drill assembly, drill string and method for connecting drill strings together which offers better durability than earlier such drill assemblies, drill strings and methods.
It is also an object of the invention to provide an improved rotary-vibratory drill assembly, drill string and method for connecting drill strings together which improves the economy of drilling operations compared with earlier drill assemblies, drill strings and methods.